single stage vs 2 stage ammonia systems

[mharper]

I have been asked to put together a spreadsheet to calculate when to use either single to 2 stage compressors on ammonia systems. I can't seem to find much other than an article in this years iiar convention book that says you should have other considerations for calculating... basically, I need to know if anyone has done this, or where to find the equations I need or anything that will help.

 

[chillin]

Stoecker (Industrial Refrigeration, ISBN 0-912524-42-1) suggests that two stage compression should seriously considered for evaporator temperatures below about -20°C (-4°F).

In practice, the decision to utilize 2-stage compression has alot to do with the load at both suction temperatures. If you have a large discrepancy between high and low side loads (e.g. really large freezer and small dock or really large cooler and small freezer), then a single stage economized system may provide enough energy efficiency to rule out 2-stage.

When the loads are significant and matching in scale, then a two-stage system may be more appropriate, given the additional first-cost of this type of system.

It will be difficult to prepare a spreadsheet that would truly take into account all of the variables, esp. if it is a true ROI evaluation. It would be useful to simulate a few different system types to see the trends, but every project should be evaluated on it's own merits before settling on a particular design. IIAR is a good start for contacting qualified refrigeration engineers to assist you (I am also a member).

-chillin

 

[mharper]

Thanks for the info. I have looked into IIAR literature recently to find some answers. One of the guys I work with is also a member/auther of IIAR. He tends to agree with you that each case needs to be evaluated. Hank Bonar suggested a spreadsheet at this year's IIAR convention, and discusses it somewhat in the manual. However, there is not enough information given there to actually create this sheet. My supervisor saw that spreadsheet in the book and gave it to me saying "I want this", which has prompted me to seek outside assistance.